The present disclosure relates to a fixing device fixing a toner image onto a sheet and an image forming apparatus including this fixing device.
Conventionally, an image forming apparatus includes a fixing device fixing a toner image formed on a recording medium, such as a sheet. The fixing device includes a fixing member heating the toner image and a pressuring member pressuring the toner image to the recording medium, and includes a temperature sensing part, such as a thermopile, sensing temperature of the fixing member in order to control heating of the fixing member. The temperature sensing part includes an infrared sensing element sensing infrared rays radiated from the fixing member and a condensing member condensing infrared rays to the infrared sensing element, and is arranged in a non-contact state with the fixing member. On the basis of the result of sensing by the infrared sensing element, detection temperature of the fixing member is calculated.
However, if the temperature sensing part is located at a position receiving an effect of heating of the fixing member, it is feared that temperature of the fixing member is erroneously detected. For example, because the condensing member is located near external environment of the temperature sensing part and easily receives the effect of heating of the fixing member in comparison with the infrared sensing element, temperature difference between the infrared sensing element and the condensing member may occur. In a case where such difference occurs, for example, if the infrared ray from the condensing member acts on the infrared sensing element to affect the result of sensing, the detection temperature may not be accurately calculated.
Moreover, the fixing device may be configured that a condensing mirror located along a longitudinal direction of a heating roller (the fixing member) condenses an infrared radiation light as temperature information of the heating roller and a reflecting mirror reflects the temperature information of the heating roller condensed by the condensing mirror to one side in an axial direction of the heating roller. In such a fixing device, a temperature sensor (the temperature sensing part) is located at a position where temperature does not become high (i.e., a position is not affected by heating of the fixing member) to input temperature information of the heating roller reflected by the reflecting mirror.
Further, the fixing device may be configured to include a thermopile (the temperature sensing part) located in a non-contact state with a heat roller (the fixing member) and the thermopile detects temperature of the heat roller on the basis of infrared rays radiated from the heat roller. This fixing device includes a direct measuring thermistor, in separation from the thermopile, detecting temperature of the heat roller. This fixing device corrects temperature detected by the thermopile on the basis of temperature detected by the direct measuring thermistor.
In the fixing device, in order to locate the temperature sensing part sensing temperature of the fixing member at the position not affected by heating of the fixing member, as mentioned above, it is necessary to provide a transmitting member, such as the condensing mirror and the reflecting mirror. Therefore, in comparison with a device not including the transmitting member and a mounting mechanism for the transmitting member, the number of components and component cost are increased, and then, enlargement and complication of the device are incurred in order to secure a mounting space for the transmitting member.
In addition, in order to correct detection temperature of a non-contact type temperature sensing part, such as a thermopile, in a case where other contact type temperature sensing part, such as a direct measuring thermistor, is arranged as mentioned above, cost may be increased in comparison with a device without the other contact type temperature sensing part and a mounting mechanism for the other contact type temperature sensing part.